1. Field of the Invention
The present invention relates to vehicle steering devices that input a steering force to'link mechanisms so as to turn left and right wheels.
2. Description of the Related Art
Generally, when a steering gear ratio is set to a low value (i.e., when the steering gear ratio is set to a so-called quick state in which the wheels are turned relatively fast in response to steering of a steering wheel) in a vehicle steering device, maneuverability in a low-speed running mode is enhanced, whereby the driver's driving load can be reduced. At the same time, when the steering gear ratio is set to the quick state, the vehicle's responsiveness to turning in a high-speed running mode becomes excessive, possibly increasing the driver's driving load.
In contrast, when the steering gear ratio is set to a high value (i.e., when the steering gear ratio is set to a so-called slow state), the vehicle's responsiveness to turning in the high-speed running mode becomes moderate, whereby the driver's driving load can be reduced. At the same time, when the steering gear ratio is set to the slow state, maneuverability is deteriorated in the low-speed running mode, possibly increasing the driver's driving load.
As countermeasures against these problems, for example, Japanese Unexamined Patent Application Publication No. 1994-227423 (referred to as “Patent Document 1” hereinafter) and Japanese Patent No. 3994780 (referred to as “Patent Document 2” hereinafter) disclose technologies for achieving both the properties required in the steering device for the high-speed running mode and the properties required in the steering device for the low-speed running mode by changing the steering gear ratio using a motor.
However, because a motor is used for changing the steering gear ratio in the technologies disclosed in Patent Documents 1 and 2 described above, the structure becomes complex, possibly leading to a significant increase in manufacturing costs. In addition, in the technologies disclosed in Patent Documents 1 and 2 described above, a slight delay in response occurring due to control performed for changing the steering gear ratio can possibly result in deterioration of the driver's operational feel.